What do you get? A free hat, naturally - of the party variety. A free noisemaker - which you can use as much as you like today and then not again for another year. Plus free cake and gifts (remodelled health kits and ammo packs).

Valve has also turned on Birthday mode - meaning levels get a "Happy Birthday Team Fortress" beach ball.

The Celebration Gib is also active, which turns exploded players' blood and guts into showers of balloons and confetti.

Team Fortress, a mod for id Software classic Quake, was followed by Team Fortress 2 in 2007. Valve has been tinkering with it ever since, and earlier this year made it free-to-play.